Generally, within the technical field of shock absorbers that include pilot valves, a pressure regulator, i.e. a valve arrangement, is used to control a flow of damping medium between a compression chamber and a rebound chamber during a reciprocal motion of a piston in a damping medium filled chamber of the shock absorber. The piston, via a piston rod, is connected either to a wheel or a chassis, whereas the chamber is connected to one of the wheel or chassis that the piston is not connected to. During a compression stroke the piston moves axially in a direction towards the compression chamber and thereby pressurizes the damping medium in the compression chamber. During a rebound stroke, the piston moves axially towards the rebound chamber, i.e. in the opposite direction, and thereby pressurizes the damping medium in the rebound chamber. In accordance with the function of the shock absorber, the pressurized damping medium needs to be transferred from the pressurized chamber to the other chamber, i.e. from the compression chamber to the rebound chamber or vice versa. The flow of damping medium needs to be controlled to obtain a damping effect of the piston and thus the shock absorber, i.e. to damp relative motion between the wheel and chassis.
The control of the pressure in the flow of damping medium in the shock absorber depends on the pressure created by the pilot valve. Pressure regulators in shock absorbers are usually provided with an axially movable or deflectable valve member, such as a washer, cone or shim that acts against a seat part. The pressure control is achieved by equilibrium or balance of forces, for example equilibrium between a pressure and/or flow force acting on the valve member in one direction and counteracting or opposing forces, such as one or more of a spring force, friction force or pilot pressure force acting on the valve member on the opposite direction. When the piston of the shock absorber moves at a certain speed such that the pressure and/or flow force become greater than the opposing or counteracting forces, the movable valve member is forced to open. Thus, the movable valve member is forced to open at a stroke defined as a function of the flow produced by the pressure acting on the regulating area of the pressure regulator.
Valve arrangements of the pressure regulating type described above may use an electrically controlled pilot valve to control a pilot pressure acting on the axially movable or deflectable valve member. One example of such an arrangement is disclosed in U.S. Pat. No. 5,934,421 where a spool valve arrangement is provided which, by moving the spool according to an electric current supplied to an actuator, directly and electrically adjusts or controls a restriction on a pilot fluid flow out from a pilot chamber. The pilot pressure acts on the deflectable valve member and thus in turn controls the pressure regulating effect on a main flow. The pilot chamber is furthermore in fluid communication with one of the working chambers of the shock absorber such that the pilot fluid flow also constitutes a bypass fluid flow between the working chambers of the shock absorber. A disadvantage with the disclosed arrangement is that the disclosed spool valve geometry only allows relatively small pilot fluid flows out from the pilot chamber. Therefore, the control of the pilot pressure, and consequently the overall control of the damping force, may be delimiting and inadequate, in particular at higher piston or flow speeds. Another disadvantage with the disclosed arrangement is that the, due to the spool valve design, control of the pilot pressure and the restriction on the bypass passage is sensitive to viscosity of the hydraulic fluid and temperature.